Complex cellular behavior is required to form an embryonic blood vessel de novo. In addition to the assembly mesodermal cells into a precise pattern, a functional vessel requires endothelial cells and vascular smooth muscle. It is likely that the mechanisms which lead to vessel formation will include cellular interactions with ECM, cell;cell adhesions, and cellular modulation by growth factors. How all these components interact in a coherent manner to produce a functional blood vessel from seemingly undifferentiated splanchnic mesoderm is unknown. The working hypothesis of this proposal is that the pivotal mechanisms which define vessel morphogenesis and differentiation are cell contact and adhesion. According to this hypothesis, molecular recognition mechanisms functioning at the cell surface mediate cell contact with specific structural features in the extracellular matrix (ECM) and on other cells. This solid-phase of recognition sites represents a "morphogenic code" which determine the behavior and differentiation of cells during development. Appropriate cell contact and adhesion are required for multiple cellular activities these include: cell shape-change, commencement/cessation of motile activity, exertion of tractional forces, proliferation, regulation of the differentiated state, and response to growth factors. We propose to investigate this hypothesis as it relates to vasculogenesis, that is, we will examine blood vessel formation de novo from splanchnic mesoderm. Specifically, we will: 1) Inject biological probes (antibodies, synthetic ligand peptides and growth factors), which are likely to perturb cell contact/adhesion, into the vasculogenic region(s) of early avian embryos (1-10 somites). 2) Determine the temporal expression and spatial distribution of ECM components, ECM receptors, and cell;cell adhesion molecules during vasculogenesis. 3) Begin studies to determine the cellular lineage of vascular smooth muscle and 4) Establish an in vitro model of vasculogenesis. The extent to which the molecular mechanisms of cell contact and adhesion can be defined, will aid in the design of therapies which lead to tissue repair and remodeling.